This project studies retroviruses produced by lymphoma cells and thymic epithelial cells of AKR mice. The viruses are cloned and tested for host range, leukemogenesis in several mouse strains. Cloned leukemogenic viruses are evaluated by oligonucleotide and tryptic peptide mapping. Thymic epithelial cells from mice of various ages are cultured and their functional properties, as well as virus production, tested.